Brain, Vol 120, Issue 12 2131-2140, Copyright © 1997 by Oxford University Press
L Schols, G Amoiridis, H Przuntek, G Frank, JT Epplen and C Epplen
Friedreich's ataxia is an autosomal recessively inherited neurodegenerative
disorder caused by expansions of an unstable GAA trinucleotide repeat in
the STM7/X25 gene on chromosome 9q. We studied the (GAA)n polymorphism in
178 healthy controls and 102 patients with idiopathic ataxia. The repeat
size ranged from 7 to 29 (GAA)n motifs on normal chromosomes and from 66 to
1360 trinucleotide repetitions in Friedreich's ataxia patients. Meiotic
instability of expanded alleles was observed without significant
differences in maternal and paternal transmissions. Thirty-six of 102
patients were typed homozygous for expanded (GAA)n alleles. Twenty-seven of
these presented with the typical Friedreich's ataxia symptoms and nine
patients with an atypical Friedreich's ataxia phenotype. Before molecular
genetic diagnosis had been performed seven of these patients had been
classified as early onset cerebellar ataxia and two as idiopathic sporadic
cerebellar ataxia of late onset. In contrast, in one family with typical
Friedreich's ataxia phenotype we did not find an expanded allele; this
suggests that there can be either point mutations in the X25 gene on both
chromosomes or locus heterogeneity in Friedreich's ataxia. The phenotypic
spectrum of Friedreich's ataxia is much broader than considered before.
Early onset, areflexia, extensor plantar responses and reduced vibration
sense should no longer be considered essential diagnostic criteria of
Friedreich's ataxia. In comparison with the non- Friedreich's ataxia group
hypertrophic cardiomyopathy seems to be the only symptom specific for
Friedreich's ataxia. However, it is not obligatory. The phenotype is
significantly influenced by the number of GAA repeats with close
genotype-phenotype relationships when the smaller of the two alleles is
considered. Repeat length correlated inversely with age at onset, onset of
dysarthria and progression rate. In conclusion, molecular genetic analysis
appears mandatory for the diagnosis and genetic counselling of Friedreich's
ataxia. The molecular genetic test should be applied not only to patients
with typical Friedreich's ataxia phenotype but also in all cases of
idiopathic autosomal recessive or sporadic ataxia.
ARTICLES
Friedreich's ataxia. Revision of the phenotype according to molecular genetics
Department of Neurology, St Josef Hospital, Bochum, Germany.
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